I don't think I can make token of instants and sorceries, but maybe it's possible to make tokens of enchantments that could perform the roles of instants and sorceries.

The gameplay is (in my experience) actually better if the activation of a rune also has a small mana cost attached to it. I would generally go for ", Sac".

It's also much easier to appropriately cost such tokens over the always active ones which can be deceptively oppressive.

Yeah, I later remembered Clue tokens, which would be something of a precedent, so an activated mana cost makes sense.

However, clues had a specific word, investigate, for their creation. I think (((Lord of Ash))) could be fine if something like "carve a red rune" was written, and then had the reminder text for the red rune as it is now.

This would actually be a good use for a secondary deck; similar to contraptions.

Then you could just say "Put a green rune into play (from your runes deck)" and have done.

Whether such a mechanic is worth the pain, though, is a different question. Is it really worth it, just to have the chance to trigger hanging around after this creature dies?

An extra deck sounds unnecessary. As stated in the comments for Lord of Ashes the rune of a color only does a single thing, so all green runes would be this ability. With green, I'd hope a player would drop another creature.

I think Vitenka is suggesting a variant of the mechanic where you can add more utility to the runes by allowing the Runes/rune creator interaction to work more like a sideboard/Wish interaction.

I actually think that's neat and I have explored similar design space for draft-mechanics.

It certainly would be cool to have an option to just be prepared enough to have a Seal of Primordium Rune in addition to the Seal of Strength Rune. Though that requires more thought put into the way to prepare explosive runes for the day.